I create a sprint, add user stories to it, then start the sprint. But some of the user stories unexpectedly fall back onto the product backlog. The "Sprint" field in these user stories is correctly set to the current sprint, but they are on the product backlog. I moved them back onto the sprint and confirm the "Sprint scope will be affected by this action" warning, but next time I look they are on the product backlog again. The only thing I see in common is that these user stories were once on a previous sprint, but taken off or not completed by the end of the sprint.